Languageclient-neovim: LC hangs when used with deoplete using any lang server

[Tylerc230]

• Did you upgrade to latest plugin version?
  yes

• Did you upgrade to/compile latest binary? Run shell command
  bin/languageclient --version to get its version number.

• (Neovim users only) Did you check output of :checkhealth LanguageClient?
  yes

• Did you check [troubleshooting]?
  yes
  

  Describe the bug

When I type a partial name of a symbol in a file type with a supported language server, the auto complete menu from deoplete doesn't show and the computer fan spins up. I tracked it down to self.vim.command("let {} = []".format(COMPLETE_OUTPUTS)). When I comment out this line, it behaves properly.
It appears that the deoplete gets caught in a loop b/c LC's source never returns any candidates.

Environment

• neovim/vim version (nvim --version or vim --version):]
  neovim 0.4.2
• This plugin version (git rev-parse --short HEAD):
  LC: 4fd272b
  deoplete: 9856613
• This plugin's binary version (bin/languageclient --version):
  languageclient 0.1.154 4fd272b0dbf96d7a4fd5d9d840780638f514aa7c
• Minimal vimrc content (A minimal vimrc is the smallest vimrc that could
  reproduce the issue. Refer to an example here):
  ```call plug#begin()
  Plug 'autozimu/LanguageClient-neovim', { 'branch': 'next', 'do': 'bash install.sh' }
  Plug 'Shougo/deoplete.nvim', { 'do': ':UpdateRemotePlugins' }
  call plug#end()

let g:LanguageClient_serverCommands = { 'swift': ['sourcekit-lsp'] }

let g:deoplete#enable_at_startup = 1
```

• Language server link and version:
  sourcekit-lsp: /Library/Developer/Toolchains/swift-DEVELOPMENT-SNAPSHOT-2019-09-16-a.xctoolchain
  pyls: ?

To Reproduce

Steps to reproduce the behavior:

1. Create/Fetch example project ...
2. Start vim, nvim -u min-vimrc.vim ...
3. Edit a source file where one would expect autocomplete to occur.

Current behavior

Notice: no autocomplete appears and laptop fan spins up

Expected behavior

Deoplete autocomplete menu to appear

Screenshots

If applicable, add screenshots to help explain your problem.

Additional context

This occurred after a recent update to both plugins.

[pappasam]

I'm also experiencing this issue, with both Neovim 0.4.0 and Neovim 0.5.0 (on Linux Mint 19.2).

Although I don't understand the cause, there have been lots of changes to Deoplete's codebase recently. The issue goes away when I pin Deoplete's version to the latest GitHub release/tag. If using minpac, this is done with 'rev':

  call minpac#add('Shougo/deoplete.nvim', {
        \ 'rev': '*',
        \ 'do': 'UpdateRemotePlugins',
        \ })

I think there's an issue with Deoplete's recent updates regarding async operations. If the next stable release is still broken, we can open an issue there.

[kchibisov]

I'm not sure if its 100% related thing, but I've noticed that deoplete completions from LC stops working on this deoplete commit. I'm not sure who is responsible for this ( LC should adopt or deoplete introduced a regression), but can someone try deoplete commit I've mentioned above and the one before it?

[Tylerc230]

Recently deoplete added some docs that say not to use is_async

is_async        (Bool)
            If the gather is asynchronous, the source must set
            it to "True". A typical strategy for an asynchronous
            gather_candidates method to use this flag is to
            set is_async flag to True while results are being
            produced in the background (optionally, returning them
            as they become ready). Once background processing
            has completed, is_async flag should be set to False
            indicating that this is the last portion of the
            candidates.

            Note: The feature is deprecated and not recommended.
            You should use callback system by
            |deoplete#auto_complete()| instead.

I tried to figure out how auto_complete() works but didn't find much. vim-lsp seems to be using it in their latest releases.

[Shougo]

@kchibisov Please test the latest version of deoplete.

@Tylerc230 Yes. is_async is not recommended for plugins.

[Shougo]

https://github.com/Shougo/deoplete.nvim/issues/1024

[kchibisov]

@Shougo latest master doesn't work for me. Deoplete shows it's own completions, however completions from LC are missing (LC is working, since it's showing error messages an so on).

[Shougo]

OK. This is deoplete problem.
I will fix it later.
Please wait.

[kchibisov]

@Shougo no problem, thx for reply here.

[Shougo]

I have fixed the problem in the latest version of deoplete.
Please update it.

[Shougo]

The issue should be closed.

[Tylerc230]

Confirmed it works. Thank you !

[Shougo]

@Tylerc230 Please close the issue.